Effects of presynaptic mutations on a postsynaptic Cacna1s calcium channel colocalized with mGluR6 at mouse photoreceptor ribbon synapses.
OBJECTIVE
Photoreceptor ribbon synapses translate light-dependent changes of membrane potential into graded transmitter release via L-type voltage-dependent calcium channel (VDCC) activity. Functional abnormalities (e.g., a reduced electroretinogram b-wave), arising from mutations of presynaptic proteins, such as Bassoon and the VDCCalpha1 subunit Cacna1f, have been shown to altered transmitter release. L-type VDCCalpha1 subtype expression in wild-type and mutant mice was examined, to investigate the underlying pathologic mechanism.


METHODS
Two antisera against Cacna1f, and a Cacna1f mouse mutant (Cacna1fDeltaEx14-17) were generated. Immunocytochemistry for L-type VDCCalpha1 subunits and additional synaptic marker proteins was performed in wild-type, BassoonDeltaEx4-5 and Cacna1fDeltaEx14-17 mice.


RESULTS
Active zone staining at photoreceptor ribbon synapses with a panalpha1 antibody colocalized with staining for Cacna1f in wild-type mouse retina. Similarly, in the BassoonDeltaEx4-5 mouse, residual mislocalized staining for panalpha1 and Cacna1f showed colocalization. Unlike the presynaptic location of Cacna1f and panalpha1 antibody staining, the skeletal muscle VDCCalpha1 subunit Cacna1s was present postsynaptically at ON-bipolar cell dendrites, where it colocalized with metabotropic glutamate receptor 6 (mGluR6). Surprisingly, Cacna1s labeling was severely downregulated in the BassoonDeltaEx4-5 and Cacna1fDeltaEx14-17 mutants. Subsequent analyses revealed severely reduced ON-bipolar cell dendritic expression of the sarcoplasmic reticulum Ca(2+) ATPase Serca2 in both mouse mutants and of mGluR6 in the Cacna1fDeltaEx14-17 mutant.


CONCLUSIONS
Presynaptic mutations leading to reduced photoreceptor-to-bipolar cell signaling are associated with disturbances in protein expression within postsynaptic dendrites. Moreover, detection of Cacna1s and Serca2 in ON-bipolar cell dendrites in wild-type animals suggests a putative role in regulation of postsynaptic Ca(2+) flux.
Specht
Dana
D
Department of Biology, Animal Physiology, University of Erlangen-Nuremberg, Erlangen, Germany.
Wu
Shu-Biao
SB
Turner
Paul
P
Dearden
Peter
P
Koentgen
Frank
F
Wolfrum
Uwe
U
Maw
Marion
M
Brandstätter
Johann Helmut
JH
tom Dieck
Susanne
S
D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D015220:Calcium Channels / Q000235:genetics* / Q000378:metabolism*; D020746:Calcium Channels, L-Type; D020013:Calcium Signaling; D000939:Epitopes / Q000276:immunology; D005260:Female; D019084:Fluorescent Antibody Technique, Indirect; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D016253:Microscopy, Immunoelectron; D010446:Peptide Fragments / Q000737:chemistry / Q000276:immunology; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000378:metabolism*; D011817:Rabbits; D018094:Receptors, Metabotropic Glutamate / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017384:Sequence Deletion; D012756:Sheep; D009435:Synaptic Transmission / Q000502:physiology*
C412223:CACNA1S protein, human; C480684:Cacna1f protein, mouse; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D000939:Epitopes; D010446:Peptide Fragments; D018094:Receptors, Metabotropic Glutamate; C113897:metabotropic glutamate receptor 6